In electrophotography, generally, an image with high quality is obtained by charging a surface of a photoconductor drum by corona discharge etc., forming an electrostatic latent image by exposure using laser etc., developing the electrostatic latent image with a toner to form a toner image, and transferring the toner image to a recording medium. The toner used for forming toner images is typically one produced by mixing a binder resin such as thermoplastic resin with a colorant, a charge control agent, a release agent, and the like, then the mixture is further mixed and kneaded, pulverized, and classified to form toner base particles with an average particle diameter of 5 μm or more and 10 μm or less. In order to provide flowability to the toner, to perform charge control of the toner, and to improve easiness of cleaning of the toner from the surface of the photoconductor drum, inorganic fine particles such as those of silica or titanium oxide are externally added to the toner.
However, the inorganic fine particles typically tend to be negatively charged, and silica exhibits a strong negative chargeability in particular. Consequently, when the inorganic fine particles are applied to a positively chargeable toner, inorganic fine particles on which surface a positively chargeable polar group has been introduced are used. When silica fine particles are actually used as an external additive, dry silica fine particles in which a positively chargeable polar group such as amino group has been introduced into the silica using an aminosilane coupling agent and a hydrophobizing treatment has been further performed using a hydrophobizing agent, wet silica fine particles in which a hydrophobizing treatment has been performed using a silicone oil, or the like have been used.
Such silica, into which a positively chargeable polar group has been introduced and subjected to the hydrophobizing treatment, is often externally added to toner base particles in order to impart flowability or a proper charging property to the toner; however, in a case of using silica with a lower particle diameter, the silica may be embedded into toner base particles depending on the size. When such an external additive is embedded into toner base particles, flowability of the toner may degrade and charged amount of the toner may be out of a desired range, thus it may be difficult to obtain images with desired image densities.
For the countermeasure to these problems, fumed silica of which primary particles are bonded and agglomerated by a covalent bond and which is produced by a flame hydrolysis method may be used in order to provide flowability to the toner and to suppress embedment of the silica into the surface of the toner. In regards to the toner externally added with silica composed of agglomerated primary particles, a toner is proposed in which flowability may be imparted to the toner and embedment of an external additive into the toner may be suppressed by use of silica having such a configuration that fine particles of 6 or more and 500 or less are covalently bonded like a chain or branch.
However, in the two-component developer where a positively chargeable toner (hereinafter, also merely referred to as “toner”) and a carrier are mixed, when the toner and the carrier are stirred, cracks may occur in the silica agglomerates due to a strong stress on the toner. Here, in a case in which a positively chargeable polar group has been introduced to the silica or the silica has been subjected to a surface treatment such as hydrophobization, and then cracks have occurred in the silica agglomerates, a positively chargeable toner of inferior charge may occur by an effect of exposure of non-surface-treated silica surface. When an inferior charge occurs in the toner, image defects such as fog may occur in the resulting images.
The present disclosure has been made in view of the problems described above; and it is an object of the present disclosure to provide a two-component developer in which lowering of image densities of resulting images below a desired value can be suppressed and occurrence of image defects such as fog can be suppressed even in cases of printing for a long period.
The two-component developer of the present disclosure includes a positively chargeable toner and a carrier. The positively chargeable toner is composed of an external additive and toner base particles containing at least a binder resin, a colorant, a charge control agent and a release agent. The external additive contains at least silica. The silica has an average primary particle diameter (X) of 12 nm or more and 30 nm or less. An average agglomeration ratio (Y2/Y1) of the silica between an average agglomeration number (Y1) on a surface of the toner base particles and an average agglomeration number (Y2) on a surface of the carrier is 0.97 or more. The average agglomeration number (Y2) on a surface of the carrier of 5 or more and 15 or less.